1. Field of the Invention
This invention relates to high density, single pass heat exchangers especially useful for drying fragmented moisture-bearing products such as fibrous materials in the nature of bakery wastes, alfalfa, peat moss and wood products.
2. Description of the Prior Art
Drying of large volumes of fragmented fibrous materials has long been carried out in heat exchangers consisting of one or more elongated, generally horizontally oriented drums. Hot gases are caused to flow through each to remove moisture from the material by heat exchange between the hot gases and the fibrous product. Generally speaking, a burner is disposed to direct hot products of combustion directly into the inlet of the drum which also receives the moisture-bearing material to be dried. After removal of the requisite amount of moisture from the material, it is directed into a collector or other receiving means at the outlet of the heat exchange drum. Oftentimes, a blower or equivalent device is associated with the drum to increase the rate of flow of hot gases through the exchanger.
Alfalfa, for example, has long been dried in horizontally positioned heat exchange drums which typically have been from about 8 to approximately 12 feet in diameter and 20 to 40 feet long. As each drum was rotated about its longitudinal axis, hot burner gases were directed into the inlet and airflow was increased by a blower connected to the outlet of the drum. Initially, single pass drums were the dryer of choice principally because of economic considerations. The manufacturing cost of the dryer was a more important overall consideration on an amortized basis than the amount of fuel burned because of relatively low natural gas prices. For many years, the alfalfa dehydration industry advertised that dehydrated alfalfa was a superior product because it was produced from alfalfa that was supplied to the dehydrated plant substantially at fresh cut moisture levels with water then evaporated from the product to its final desired dry level of no more than about 8 to 10%.
Even during the era of relatively low fuel prices, single pass evaporators in the alfalfa dehydration industry were supplanted in many instances by so-called three-pass dryers which permitted processing of higher volumes of the ground alfalfa in essentially the same overall ground space occupied by then existing single pass heat exchangers. In three-pass dryers, material was directed along a generally S-shaped path of travel thereby providing a longer path and shorter residence time in the dryer by virtue of increased hot gases velocities.
Single pass dryers have previously been employed having inwardly directed internal flighting in the drum which caused the material conveyed through the dryer to be lifted up somewhat and dropped back into the hot gas stream rather than simply resting at the bottom of the drum as it was rotated about its axis. An exemplary design in this respect is disclosed in applicant's U.S. Pat. No. 4,183,208 which incorporated secondary flighting in the central part of the drum for the purpose of enhancing heat exchange between the hot gases directed through the drum and the product to be dried.
In order to prevent hot gases from being blown directly through the drum from one end to the other, single pass dryer designers in the past have resorted to the use of a transverse plate or plates in the drum to obstruct the flow of hot products of combustion. The net result of such construction inevitably was to decrease the capacity of the dryer while at the same time interfering with uniform temperature control and preventing maintenance of constant material flow rates through the heat exchanger.
Three-pass dryers, on the other hand, were more expensive than single pass dryers but found favor because of the decreased product residence time necessary to assure adequate drying and more stable temperature control performance.
As fuel prices have risen, dehydration plant operators for example have become increasingly more concerned about fuel costs and have significantly retreated from a position that alfalfa should be introduced into the dryer at fresh cut moisture levels approaching 85%. More and more reliance has been placed on sun curing with the alfalfa introduced into a dryer being no higher than about 35 to 75% moisture. Significantly higher fuel costs have resulted in a substantial retrenchment in the alfalfa dehydration business with less efficient plants being closed down rather than updated.
As plants have closed, those still in business have sought ways to rehabilitate their equipment by increasing efficiency so that the facilities will accommodate greater and greater demand in order to permit continuing operation. New plants also require that a highly efficient process be provided to effectively compete when the overall costs of building a totally new plant are factored into the equation.
The rotary kiln illustrated a described in U.S. Pat. No. 4,753,019 and which was designed to dry calcium carbonate sludge, would not be useful to remove moisture from a material such as alfalfa or the like. The '819 patent mixing rods within the drum would not provide adequate agitation of the material to be dried, and especially would not function to lift the material and provide a shower thereof which falls through the hot drying gases.
The asphalt dryer of U.S. Pat. No. 4,338,732 has internal flighting for lifting of the product, but does not have adequate means to assure turbulent flow of hot gases through a product such as alfalfa, and there would be inadequate residence time of the fragmented material in the drum to reduce the moisture content of alfalfa or a similar product to a required level.